Protein folding in vivo is typically more efficient than in vitro folding. The cellular factors responsible for fast, efficient in vivo folding are therefore coming under increasing scrutiny as scientists attempt to express novel proteins in bacteria. One factor present in all in vivo folding reactions, yet absent from in vitro experiments, is the ribosome. Not only is the ribosome the first cellular component that the nascent polypeptide chain encounters, it has also been shown to interact with portions of the nascent chain considerably after peptide bond synthesis. As interactions with the ribosome have the potential to dramatically affect the initial conformations Of the newly- synthesized polypeptide, I propose to characterize the interactions of the ribosome with phage P22 tailspike, a trimeric beta-coil protein. Tailspike has the distinction of being one of the few proteins whose folding pathway is well-characterized both in vivo and in vitro, and several methods have been established to characterize its folding intermediates. Recently, it has been shown that stable complexes of ribosomes with nascent tailspike chains can be isolated, demonstrating the tractability of this system. The goal of this research is to characterize the conformational features of the ribosome-bound tailspike chains, and compare these features with those seen for the in vitro folding pathway. In addition, I will characterize site on the ribosome that interacts with the nascent polypeptide chains.